How To Use WheelWise Tire Fitment Tool (Instructions)

However, it isn’t always a stress free experience either trying to determine just what tire size to run. How will it fill out the wheel well? Will it clear my suspension and frame components? We’ve been there ourselves, and that is why Speedway Motors engineers sharpened their pencils and came up with the latest product in our WheelWise™ product line of fitment tools—the patent pending WheelWise™ Tire Fitment Tool. This tool easily provides a way to simulate various tire heights and widths within your vehicle’s wheel opening. It is a “sister tool” to our line of WheelWise Wheel Fitment Tools. Whereas the original WheelWise™ Wheel Fitment Tool is designed to measure and determine wheel offset with a known tire size, the WheelWise™ Tire Fitment Tool can be used to test multiple tire sizes to determine the best fit. Then you can use that tire size calculated by the WheelWise™ Tire Fitment Tool in conjunction with the Speedway Motors wheel fit tool to provide 100 percent confirmation of your wheel and tire package. Let’s go over the WheelWise™ instructions for our Tire Fitment Tool and how to use the calculations required to provide an accurate tire size.

It may be necessary, depending upon your disc brake caliper’s overhang to the rotor hat, to remove your disc brake caliper and/or caliper mounting bracket to provide sufficient clearance to manipulate the tool through the arc of the wheel opening to check for full and proper fitment. If you do need to temporarily remove the brake caliper, be sure to support it properly and not let it hang by the brake hose. Lastly, you will need to load the suspension with a floor jack (to simulate ride height). A better solution is to remove the spring or coilover shock to allow full, unobstructed articulation of the suspension to verify proper tire clearance and full compression.

How To Read a Tire Sidewall

Tire sizes come in either metric or SAE sizing. These tire sizes are often converted from one type to another, such as when replacing bias ply tires with metric radials. Throughout the rest of this article and the math we show, certain tire terms like tire width and aspect ratio are used. These are easily read from the tire sidewall information of the tire you are wishing to fit (and can be sourced from tire manufacturer websites as well).

Metric tire sidewall example of a 275/60R15: Tire width in mm (275) / tire aspect ratio percentage (60) R for radial construction, wheel diameter in inches (15)

SAE tire sidewall example of a 28x10.5-16 Tire diameter in inches (28) X tire width in inches (10.5) - wheel diameter in inches (16)

1. Begin by calculating your desired tire size's sidewall height using either SAE or metric sizing calculation provided below (note, some tire manufacturers provide this information on their website product page, saving you the math steps).

2. Set the tire tool’s width and sidewall sliders to the specs provided in your math and then slide the tool spacing in and out to verify proper tire clearance to all frame, suspension, and body components. Once the desired tire spacing is found, secure the tire spacing adjustment.

3. Using the tire spacing adjustment measurement, along with the tire width and sidewall slider measurements, you can now calculate your wheel’s backspacing and offset. Find the recommended wheel width for the tire size you are using (again, most manufacturers provide this data on their website) giving you wheel width, backspace, diameter, and offset to search for wheels that will fit your application and desired tire size.

Math Examples to Fit a 275/60R15:

1. Calculate Sidewall Height: Tire aspect ratio divided by 100, multiplied by tire width in mm = Sidewall Height in Millimeters

60 / 100 x 275 = 165mm sidewall height

2. Calculate Wheel Offset: Tire Width in mm divided by 2, subtract tire spacing in mm = Wheel Offset in Millimeters

275 / 2 – 137 (read from tool’s tire spacing slider) = 0.5mm offset (rounded down to 0 offset)

3. Calculate Backspacing: Tire width in inches plus wheel width in inches, divided by 2, subtract tire spacing in inches, plus .5 inch = Backspacing in Inches

10.8 + 8 (our recommended wheel width) / 2 – 5.4 (tire spacing converted inches) + .5 = 4.5-inch backspacing

Final wheel specs for our selected tire size: 15x8 wheel with 0 offset and 4.5 inches of backspacing to properly mount and place our 275/60R15 in the vehicle’s wheel opening.

Math Examples to Fit a 15x8 wheel with 4.5” backspacing and 0 offset:

1. Calculate Wheel Offset: 25.4 X [(backspacing – wheel width / 2) –.5]

25.4 X [(4.5 – 8 / 2) – .5] = 0 mm offset

Note that wheel offset is traditionally only given in millimeters

To break this math down further follow these steps to success:

• Step 1: Divide wheel width by 2 first (8" divided by 2 equals 4")
• Step 2: Subtract the result of step 1 from the backspacing (4.5" subtract 4" equals -.5")
• Step 3: Subtract .5 from the result of step 2 (-.5" subtract -.5" equals 0")
• Step 4: Finally, multiply the result of step 3 by 25.4 (0" multiplied by 25.4 equals 0mm)

2. Calculate tire spacing based on wheel offset (found in step 1) and chosen tire width that is compatible with your wheel width (tire width / 2 – offset)

275 / 2 – 0 = 137.5mm tire spacing

3. Calculate final tire size by sidewall height dimension from tool once you determine suitable tire clearance (sidewall height / tire width X 100)

165mm / 275mm X 100 = 60 aspect ratio

Final tire size for the wheels being used: 275/60R15

1. Choose your wheel diameter

18-inch wheel diameter

2. Set your tool to the wheel diameter and determine maximum tire width, sidewall height, and tire spacing for your application. Record tire height and width to calculate final tire size. For our sample fitment we’ll use 295mm tread width and 105mm sidewall height.

Measured tire width: 295mm Measured tire height: 105mm

We can now determine aspect ratio using the following calculation: (sidewall height / tire width X 100 =)

105 / 295 X 100 = 35.59 (rounded down to 35, as aspect ratio is always in a factor of 5)

3. Calculate wheel offset using your tire spacing dimension read off your tire fitment tool. For our example we’re using a reading of 130mm. (tire width / 2 – tire spacing =)

295 / 2 – 130 = 17.5mm (rounded up to 18mm offset)

4. Choose a recommended wheel width compatible with your tire size to calculate your needed wheel backspacing. You can usually find this information on the wheel manufacturer’s website. For our example we’ll be using a 10-inch wide wheel. Remember to convert from mm to inch you divide the tire width and wheel width by 25.4, as these need to be converted to inches for this step. [(tire width + wheel width / 2 – tire spacing) + .5]

(11.6 + 10 / 2 – 5.1) + .5 = 6.2 in (rounded up to 6.25-inch backspacing)

To break this math down further follow these steps to success:

• Step 1: Add tire width and wheel width first (11.6" plus 10” equals 21.6")
• Step 2: Divide the result of step 1 by 2 (21.6" divided by 2 equals 10.8")
• Step 3: Subtract tire spacing from the result of step 2 (10.8" subtract 5.1" equals 5.7")
• Step 4: Finally, take the result of step 3 and add .5” (5.7” plus .5” equals 6.2”)

You now have full wheel and tire specs for this set of measurements:

• Wheel specs: 18x10 with 6.25 inch of backspacing and 18mm offset
• Tire specs: 295/35R18

Note that final tire sizing varies between manufacturers, so one brand’s 295/35R18 may differ slightly in width or sidewall height than another brand of the same size. When possible, always confirm true measurements on the manufacturer’s website. We highly recommend buying one sample tire and rechecking fitment using the actual tire and our WheelWise™ Wheel Fitment Tool for complete verification of tire sizing fitment for your application. If you wish to size up or down your tire calculations, simply adjust the tire fitment tool, take new readings, and plug them into these calculations.

Backspacing and Offset Unit Conversions Wheels are generally dimensioned with the backspacing given in inches and the offset given in millimeters. Because both units are often used, the following two equations can be used to convert inches to millimeters and vice versa.

• Millimeter to inch formula: inches = millimeters ÷ 25.4

• Inch to Millimeter: millimeter = inches x 25.4

Tire Sizing and Reading Tire Sidewalls

Tire sizes come in the two basic styles shown below, the more common metric and the older, classic SAE.

Metric Tire Sizing: Tire width in mm / tire aspect ratio (R for radial construction) wheel diameter in inches

Example Tire Size: 275/60R15

SAE Tire Sizing: Tire diameter in inches X tire width in inches - wheel diameter in inches

Example Tire Size: 26x10.5-15

To convert between SAE and metric tire sizes, the following equations can be used:

Metric to SAE:

• Tire diameter: (Tire width in mm X tire aspect ratio / 2540 X 2) + wheel diameter = Example: 275/60R15 tire (275 x 60 / 2540 X 2) + 15 = 27.99 inches (round up to 28)

• Tire section width: Tire width / 25.4 = Example: 275 / 25.4 = 10.8 (round up to 11)

Final conversion: Our 275/60R15 metric tire is equivalent to a 28x11-15 SAE tire

SAE to Metric:

• Tire section width: Tire width X 25.4 = Example: 28x11-15 tire 11 X 25.4 = 279.4 (round up to next unit of five at 280)

• Tire sidewall: (Tire diameter – wheel diameter) / 2 = Example: 28x11-15 tire (28 – 15) / 2 = 6.50

• Tire aspect ratio: Tire sidewall / tire section width X 100 = Example: 6.50 / 11 X 100 = 59 (round up to the next unit of five at 60)

Final conversion: Our 28x11-15 SAE tire is equivalent to a 280/60R15 metric tire. In this case a “280” width tire may be difficult to source, moving up or down in 5mm increments to a 275 or 285 will provide a larger pool of tire options.

Additional Toolbox Reading

• Wheel Diameter & Fitment Guide
• Can I Mix Bias and Radial Tires
• Wheel Offset Explained
• Bias Ply vs Radials: What's Right For Your Car?
• How Big of Tires Can I Fit - Plus Sizing Guide